*If you don't want to use oil, sauté the vegetables in a little water or broth instead.
Preheat your oven to 425°F, and sprinkle a 12-inch round pizza pan with cornmeal.
Combine the flour, sugar, baking powder and salt in a bowl. Cut in the butter with two knives, a pastry blender or a food processor until the mixture resembles coarse crumbs. Add the milk gradually until a smooth, soft (but not sticky) dough forms.
Roll the dough out on a floured work surface to a 13-inch round, and place it in the pan, crimping up the edges. Set it aside.
Sauté the raw vegetables in the olive oil in three separate batches, the broccoli just until bright green and crisp-tender, and the squash and mushrooms just until softened. Set them aside to cool.
Just before the toppings are ready, bake the crust in a preheated 425°F oven for about 5 minutes. Let cool.
Spread the tomato sauce over the cooled pizza crust, sprinkle with the minced garlic, and dollop scant tablespoonfuls of ricotta at random intervals over the sauce, flattening them slightly. Mix the olives with the mushrooms and top the pizza in quarters
Bake the pizza in a preheated 425°F oven for 15 to 17 minutes, or until the crust is crisp and the cheese has melted. Cool slightly before devouring.